Amphimerycidae

The earliest taxonomic history of the Amphimerycidae was in 1804 when the French naturalist Georges Cuvier erected Anoplotherium minimum, stating that unlike with other species assigned to Anoplotherium (A. commune, A. medium, and A. minus), A. minimum lacked known postcranial fossil evidence.^[1] In 1822, he emended A. minimum to A. murinum, noting that the species was still only known from its skull unlike with other Anoplotherium species, and classified it to the subgenus Dichobune.^[a][2] In 1848, the French palaeontologist Auguste Pomel erected the genus Amphimeryx for the reclassified species A. murinus, arguing that it was close to ruminants in affinity.^[4]

In a 1891–1893 palaeontology textbook, the German palaeontologist Karl Alfred von Zittel classified Amphimeryx to the artiodactyl family Xiphodontidae.^[5] Swiss palaeontologist Hans Georg Stehlin reclassified it and the newly recognized Pseudamphimeryx to their own family, the Amphimerycidae, in 1910. He also noted that while Amphimeryx was long thought to have been closely related to Xiphodon, the possibility that both the Amphimerycidae and Xiphodon independently acquired similar anatomical traits couldn't be eliminated.^[6] In 1961, The French palaeontologist Jean Viret provided a formal diagnosis of the Amphimerycidae, describing the anatomical traits that separated the family from its relatives.^[7]

For much of the taxonomy history of the amphimerycids, their placements within or outside the Ruminantia had been disputed and still remains so today.^[8] In 1941, the American palaeontologist Edwin H. Colbert wrote about evolutionary affinities of fossil and extant ruminants, comparing Archaeomeryx to other artiodactyl genera like Amphimeryx, Hypertragulus, Gelocus, and Tragulus. He said that fossil evidence of Amphimeryx was not complete during his time of study but suggested that it may have been a primitive member of clade Tragulina but that Archaeomeryx was overall more primitive than it. He then classified Amphimeryx to its own ruminant superfamily Amphimerycoidea, separating it from other traguline superfamilies like the Hypertraguloidea and Traguloidea.^[9] His classification was followed by another American palaeontologist George Gaylord Simpson in 1945.^[10]

In 1961, Viret reclassified the Amphimerycidae and the Xiphodontidae into the artiodactyl clade Ancodonta, therefore removing the former from the Ruminantia. Similarly, American palaeontologists S. David Webb and Beryl E. Taylor in 1980 argued that the Amphimerycidae had historically been tied to the Ruminantia due to postcranial convergences but otherwise had more in common with xiphodonts than ruminants in terms of dentition. However, they chose to tentatively reclassify the Xiphodontidae and Amphimerycidae to the Tylopoda instead, although they did also suggest the possibility of them being a sister group to ruminants. On the other hand, in 1997, the American palaeontologists Malcolm McKenna and Susan K. Bell reclassified the Amphimerycidae into the Ruminantia.^{[7][11][12][8]}

According to Jörg Erfurt and Grégoire Métais in 2007, the similarities of amphimerycids with ruminants may be an instance of parallel evolution, in which amphimerycids and ruminants independently gained similar traits.^[8][13] While amphimerycids have typically been excluded from the Ruminantia due to their dental characteristics, it does not eliminate the possibility of them being the sister taxa to ruminants by the latter independently gaining longer legs and more selenodont (crescent-shaped) dentition.^[14] Its affinities, along with those of other endemic European artiodactyls, are unclear; the Amphimerycidae, Anoplotheriidae, Xiphodontidae, Mixtotheriidae, and Cainotheriidae have been determined to be close to either tylopods (i.e. camelids and merycoidodonts) or ruminants. Different phylogenetic analyses have produced different results for the "derived" selenodont Eocene European artiodactyl families, making it uncertain whether they were closer to the Tylopoda or Ruminantia.^[13][15][16]

In an article published in 2019, Romain Weppe et al. conducted a phylogenetic analysis on the Cainotherioidea within the Artiodactyla based on mandibular and dental characteristics, specifically in terms of relationships with artiodactyls of the Palaeogene. The results retrieved that the superfamily was closely related to the Mixtotheriidae and Anoplotheriidae. They determined that the Cainotheriidae, Robiacinidae, Anoplotheriidae, and Mixtotheriidae formed a clade that was the sister group to the Ruminantia while Tylopoda, along with the Amphimerycidae and Xiphodontidae split earlier in the tree.^[16] The phylogenetic tree used for the journal and another published work about the cainotherioids is outlined below:^[17]

In 2020, Vincent Luccisano et al. created a phylogenetic tree of the basal artiodactyls, a majority endemic to western Europe, from the Palaeogene. In one clade, the "bunoselenodont endemic European" Mixtotheriidae, Anoplotheriidae, Xiphodontidae, Amphimerycidae, Cainotheriidae, and Robiacinidae are grouped together with the Ruminantia. The phylogenetic tree as produced by the authors is shown below:^[15]

In 2022, Weppe conducted a phylogenetic analysis of Palaeogene artiodactyls, focusing mostly on the endemic European families. While the clade consisting of P. renevieri and A. murinus was recovered as a sister group to the other endemic artiodactyl clades, the placement of P. schlosseri rendered the Amphimerycidae paraphyletic in relation to the derived amphimerycid species and other families. He argued that the Amphimerycidae thus needs a systemic revision for which P. schlosseri would be assigned to a new genus and removed from the Amphimerycidae.^[13]

The Amphimerycidae is a family of small-sized artiodactyls whose species ranged in weight from 0.4 kg (0.88 lb) to 1.5 kg (3.3 lb) total. In a 1995 study by Jean-Noël Martinez and Jean Sudre on several Paleogene artiodactyls found that only Messelobunodon is smaller than Amphimeryx.^[18] Amphimerycids are defined in part as having an elongated snout and large orbits (eye sockets) that are widened in their backs.^[8] The dental formula of the Amphimerycidae is 3.1.4.33.1.4.3 (3 incisors, 1 canine, four premolars and three molars in each half of the upper jaw, and 3 incisors, 1 canines, 4 premolars and 3 molars on each half of the lower jaw) for a total of 44 teeth, consistent with the primitive (ancestral) dental formula for early-middle Palaeogene placental mammals.^[19][20] The incisors (I/i) are shovel-shaped and have sharp edges on their crowns.^[21] The canines (C/c) are incisiform (incisor-like in form/shape) and therefore differ little with the incisors themselves. The premolars (P/p) are elongated and may generally be separated by diastemata (gaps between teeth). The lower premolars have three lobes, or developed areas on their crowns. The upper molars (M/m) are more developed in form and are generally subtriangular in shape, although some may be more rectangular. They have five crescent-shaped (selenodont) tubercles and sometimes a partial hypocone cusp that may be present in all species.^[19][8] Amphimerycids differ from ruminants, particularly the basal clade Tragulina, in the retentions of their first premolars and their high levels of specialization in their selenodonty and number of cusps in their molars.^[22] Their dentitions more closely resemble those of xiphodonts or dacrytheriines than of ruminants.^[8] The overall selenodonty and brachyodonty (low-crowned teeth) of amphimerycids suggest that they were adapted towards folivorous (leaf-eating) dietary habits.^[18] Pseudamphimeryx and Amphimeryx, both known by multiple skull specimens, have very similar forms but differ based on a few characteristics.^[21] Amphimeryx is distinguished from Pseudamphimeryx in part by the more well-developed occipital crest.^[23] While the peak of the skull's top of Amphimeryx slopes down to its front area, that of Pseudamphimeryx appears initially concave at the occipital crest's front, ascends slightly, and then finally slopes down.^[21]

Both amphimerycid genera have especially prominent occipital and sagittal crests, the latter of which divides into two less prominent branches behind the fronto-parietal suture (suture overlapping the frontal bone and parietal bone) that extend up to the supraorbital foramen. The frontal bones of both amphimerycid genera are large and flat, being particularly sizeable in their supraorbital portions; this trait is more pronounced in Amphimeryx. The lacrimal bone of both amphimerycids, but especially in Amphimeryx, has an extensive pars facialis (underside of the orbit) and is quadrangular in shape, narrowing at its front. The orbit is large, is positioned near the back of the skull, is wide at its back area, and is more curved at its upper compared to lower edge. The optic foramen, located in the sphenoid bone, extends more forward in Amphimeryx than in Pseudamphimeryx. While the nasal bone is not as well-preserved in Amphimeryx fossils, the frontonasal suture (overlapping with the frontal and nasal bones) is implied to have formed a W shape on the skull's upper surface like that of Pseudamphimeryx. Both amphimerycid genera also have similar, although not identical, medial (or middle) positions of the infraorbital foramen in the maxilla. The palatine bones of Amphimeryx and Pseudamphimeryx are narrower at their front than back ends.^[21]

Both amphimerycid genera are best known by the "cubonavicular" bone (fused cuboid bone and navicular bone of the hind legs) recorded in multiple species; the morphology of the astragalus of P. renevieri further attests to anatomical support of the fused bone within the family.^[8][24] This trait has also been recorded in ruminants, suggesting that the amphimerycids and ruminants independently acquired the trait in an instance of parallel evolution.^[25][8] The primitive state of the astragalus sets Amphimeryx apart from ruminants; the approximately equal sizes of its trochleas (grooved pulley-like structure) and more rounded edge of its sustentacular (or supporting) facet also sets the genus apart from the Cainotheriidae.^[26] In Amphimeryx, the middle metatarsal digits III and IV are elongated and partially fused to each other while the side digits II and V are greatly reduced to small but needlelike forms. Digit III measures 50 mm (2.0 in) long while digit II is no more than 14 mm (0.55 in) long.^[8][19] These traits are similarly recorded in derived ruminants, which have tetradactyl (four-toed) feet, absent digit I, reduced digits II and V, and fused digits III and IV that make up the cannon bone (the now-extinct primitive ruminants had five-toed feet, unreduced digits II and V, and unfused digits III and IV).^[22][27] Like other artiodactyls with only two elongated digits in each foot (digits III and IV),^[28] Amphimeryx was functionally didactyl, meaning that it walked only on its two elongated toes per foot despite having four total.^[26]